This invention relates generally to a control apparatus responsive to a changing stimulus, and it also relates generally to a method of producing a control response to a changing stimulus. This invention relates more particularly, but not by way of limitation, to an apparatus for controlling the opening of a valve disposed in a tubing string in a well during a drill stem test wherein the valve is opened during a flow period of the test and closed during a shut-in period of the test. This invention also relates more particularly, but not by way of limitation, to a method of opening a bypass valve during a drill stem test.
Various stimuli can be monitored to produce a desired response. Voltage, current and frequency are examples of electrical stimuli. Chemical reactions can also be stimuli, as can mechanical phenomena such as the speed of a object. Pressure is another type of stimulus which is sometimes monitored to produce a response.
A stimulus has an identifiable characteristic, such as magnitude or rate of change, which can be detected to produce the response. For example, the rate of change of pressure is an identifiable or detectable characteristic. The rate of pressure change in an oil or gas well will be used as a specific example.
During the development of an oil or gas well, a drill stem test might be performed to determine pressure characteristics which provide important information about the ability of the well to produce hydrocarbons. During a drill stem test, a valve is opened and closed to define flow and shut-in periods during which hydrocarbons are allowed to flow to the surface or stopped from doing so. Stopping the flow allows the pressure to build up in the well. To perform a drill stem test efficiently, a suitable end of each shut-in period needs to be identified so that a valve can be opened to permit flow. During a flow period the pressure decreases to a minimum. This flow condition can generally be detected at the mouth of the well by an operator who can manually cause the valve to be closed after some time period as desired. During a closed-in, or shut-in, period, however, the pressure is not communicated to the surface so that some means is needed by which to know when to open the valve. Typically, the valve should be opened when the pressure is increasing slowly enough to indicate that steady-state maximum pressure has almost been reached (although steady state may not have actually been reached, this condition is taken as indicative of steady state and thus will be referred to as a steady-state condition). Opening the valve before this condition is reached can produce erroneous or incomplete data, and delaying opening of the valve until well after this condition wastes testing time and money.
A drill stem test can be performed using a wire line tool whereby downhole pressure is monitored and data signals are immediately transmitted to the surface on the wire line cable on which the tool is suspended in the well. This gives the operator real-time information from which he knows when to open the valve; however, wire lines have disadvantages well known in the industry. For example, there can be installation problems; the wire can break, causing time to be lost while retrieving the tool; and there can be fire and explosion hazards.
Another previous technique for performing drill stem tests includes installing pressure gauges at the bottom of a drill string. The drill string also has the necessary porting valves that can be opened and closed from the surface to establish the flow and closed-in periods necessary for the tests. Although these gauges can collect the information, they do not transmit the information to the surface on a real-time basis; therefore, this type of system does not help in determining how long the shut-in periods should be maintained. That is, without the real time information which can be provided via a wire line type of system, a shut-in period might not be maintained long enough, thereby possibly resulting in an invalid or unanalyzable test, or maintained too long, thereby unnecessarily prolonging the test and unnecessarily increasing rig time costs.
For the specific example of needing to determine when pressure reaches a steady-state condition during a shut-in period of a drill stem test, there is the need for an improved apparatus and method for sensing and providing a control signal in response to a steady-state condition of changing pressure during the shut-in period. In particular, there is the need for an apparatus and method for use downhole in a well to monitor pressure and control automatically a bypass valve to define more efficiently the end of a shut-in period and the start of a flow period of a drill stem test. Although there are these specific needs, there is also the broader need for such an apparatus and method to be applicable more generally to a control apparatus and method for producing a control response to a changing stimulus.